Effect of Incorporating Hydroxyapatite and Zinc Oxide Nanoparticles on the Compressive Strength of White Mineral Trioxide Aggregate

Statement of the Problem: Many efforts have been made to improve the properties of mineral trioxide aggregate (MTA), including the incorporation of nanoparticles. Purpose: The aim of this study was to investigate the incorporation of zinc oxide and hydroxyapatite nanoparticles on the compressive strength of white MTA (WMTA). Materials and Method: In this in vitro study, the following materials were evaluated: MTA, MTA+5% zinc oxide (ZnO) nanoparticles, MTA+10% zinc oxide nanoparticles, MTA+5% hydroxyapatite (HA) nanoparticles, MTA+10% zinc oxide nanoparticles. The compressive strength of the groups under investigation was measured on days 4 and 21 after mixing the MTA using a universal testing machine. Two-way ANOVA test was used to compare the groups and determine the significance of the effect of time and material on the compressive strength (p<0.05). Results: The highest and lowest compressive strength values were respectively measured for the second group, MTA/21 days, and the fourth group, MTA+Nano ZnO/4 days. Two-way ANOVA indicated that incorporation of zinc oxide and hydroxyapatite nanoparticles into MTA did not have a significant effect on compressive strength (p= 0.05). Compressive strength in all the groups increased over time from day 4 to day 21. However, this increase was not statistically significant (p= 0.06) except for the MTA group, which exhibited significant increase in compressive strength over time from day 4 to day 21 (p=0.007). Conclusion: Incorporation of HA and ZnO nanoparticles into MTA had no detrimental effects on its strength and these nanoparticles can be used to improve the other properties of MTA.


Introduction
Mineral trioxide aggregate (MTA) is hydrophilic cement with calcium silicate base, which is widely used in endodontics to repair perforations, in vital pulp therapy and as a surgical retrograde material due to several favorable properties, including biocompatibility and the ability to induce osteogenesis and cementogenesis [1][2][3].
One of the disadvantages of MTA is its long setting time and difficult handling [4][5]. Various methods have been proposed to improve the MTA properties, including adding materials such as sodium hydrogen phosphate (Na 2 HPO 4 ), calcium chloride, and nanoparticles [6][7][8][9].
The physical properties of an endodontic biomaterial, such as compressive strength, are important in cases where this material is subjected to occlusal forces [10].
The similar clinical conditions of this case are the application of MTA as a pulp capping material, in apexogenesis and in perforation repair [11], where the material is subjected to the force of restorative materials as well as to occlusal forces [12]. According to previous studies, the compressive strength for MTA immediately after setting is 40 MPa (Mega Pascal), which increases to 67 MPa within 21 days [13][14]. Several factors affect the compressive strength, including the type of MTA, condensation pressure, the acid etching process, the mixing technique, and the liquid mixed with MTA [12][13]. Over time, various materials have been added to improve the properties of MTA. One of the materials that have been considered in this regard in various studies is nanoparticles, including silica, silver, and silver zeolite [9,[15][16][17]. In one study, the incorporation of silica nanoparticles reduced the setting time and increased the compressive and flexural strengths of MTA [16].
Hydroxyapatite (HA) is an important biological material and is the main component of the mineral bone and teeth [18]. It is widely used in medicine and dentistry [18][19]. HA is used to improve the setting time of MTA [20] and to improve the osteogenic properties because of its biocompatibility [21].
Zinc oxide (ZnO) particles are antimicrobial substances, which have been used for many years in various dental compositions due to biological adaptation [17].
Zinc activates enzymes, which are toxic to bacteria at low concentrations, and inhibits plaque growth at higher concentrations [22].   In this study, we considered that all the cylinders were filled with the material and that the upper surface of the material was leveled with the edges of the cylinder. The cylinders that did not have this feature were excluded from the study.

Confounding variables
Since the setting condition of the material affects the

Results
The results of the descriptive statistics, i.e. means and standard deviations of the data ( to day 21(p= 0.007) (Figure 1).

Discussion
MTA is a calcium silicate-based material. It is highly biocompatible and can induce hard tissue formation due to its favorable characteristics. It has been accepted as the best standard for endodontic treatments for many years [23][24]. In spite of these desirable characteristics, long setting time and technical sensitivity of this material have prompted the researchers to add different materials, including nanoparticles, to improve some of its properties. Considering its application in the field of root canal treatment, its physical properties, including compressive strength, have been of great importance in various studies [13,[25][26], which should be reexamined after adding any material to its structure. The initial strength of MTA is 40 MPa and according to previous studies, it reaches 67 MPa in 21 days [14,27].
Compressive strength reflects the quality of the hydration process and hydration is a factor directly affecting the MTA setting; therefore, any factor affecting the hydration process also affects its physical properties [2,10,25,[28][29].
One of the materials evaluated in this study was HA, which is considered as an important biological material and the main component of the bone and teeth. The HA nanoparticles are more efficient because of fine particles, having more contact surfaces and higher solubility [30]. The HA has bone formation potential and can directly bind to the bone [31]. It is easily tolerated and integrated into the host tissue [32] and because of the lack of protein, it does not cause allergic and immune reactions [33].
In the current study, incorporation of HA nanoparti- HA is a highly active substance that results in rapid bonding due to its nano-structure and reduces the setting time [34], which can increase compressive strength on the early days (4 days after mixing).
In other studies, various nanoparticles have been added to improve the properties of MTA [6,[15][16]. In a study by Akbari et al. [16], it was found that incorporation of nano silicate particles such as the particles used in our study had a positive effect on compressive strength of MTA, but this effect was not significant.
A study by Prasad et al. [6] showed that incorporation of calcium formate, calcium chloride, and disodium hydrogen orthophosphate nanoparticles, in contrast to the nanoparticles used in our study, reduced the compressive strength of MTA. In a study by Bernardi et al. [26], calcium carbonate nanoparticles reduced the compressive strength of MTA. Another nanoparticle considered in our study was ZnO, which has been shown in many studies to have antibacterial properties [22,35].
In the group in which ZnO alone was added to MTA, the compressive strength slightly decreased although this decrease was not statistically significant.
However, in a study by Guerreiro-Tanomaru et al. [ days was selected because the initial strength is important in clinical applications and the material is initially exposed in the patient's mouth to occlusal forces.
Therefore, the material in this period undergoes favorable setting; moreover, the same period has been selected for this purpose in previous studies [15,28].
The 21-day period was also selected to study the effects of HA and ZnO in a longer period and along with a shorter period, the results were compared with the control group (without nanoparticles). Long-term strength is important for materials' resistance to occlusal force and the force created by the placement of the restorative materials [28].
A significant increase in compressive strength was observed over time from day 4 to day 21 only in the MTA group alone, which is consistent with previous studies [15,26]. In other groups, there was a slight increase in the compressive strength, which was not statistically significant.

Conclusion
According to the results of the present study, HA and